Fiber reinforced composite materials that consist of reinforcing fiber, such as glass fiber, carbon fiber and aramid fiber, and matrix resin, such as unsaturated polyester resin, vinyl ester resin, epoxy resin, phenol resin, cyanate ester resin and bismaleimide resin, are lightweight but high in strength, rigidity, shock resistance, fatigue resistance and other mechanical properties, in addition to being high in corrosion resistance, and accordingly have been used in aircraft, spacecraft, automobiles, railroad vehicles, ships, construction material, sporting goods, and many other materials in different fields. In particular, fiber reinforced composite materials composed of continuous fiber are generally used to produce high performance products, with carbon fiber and thermosetting resins, epoxy resin among others, being frequently used as reinforcing fiber and matrix resin, respectively. In a widely used process to produce fiber reinforced composite material that consists of continuous fiber and thermosetting resin, prepregs composed of reinforcing fiber and uncured thermosetting resin are produced as intermediate, followed by their lay-up and heat curing. This process, however, cannot be said to be excellent in terms of cost because it requires production of prepregs as intermediate. Compared to this, a production technique called resin transfer molding (RTM) in which liquid thermosetting resin is injected into a reinforcing fiber substrate placed in a mold, followed by heat curing to produce fiber reinforced composite material, has attracted much attention in recent years due to its high productivity in manufacturing fiber reinforced composite material. Recent reports on resin transfer molding (RTM) include SAMPE Journal, Vol. 34, No. 6, pp. 7-19 (1998). Lately, this technique is in wider use in production of aircraft material and other materials that require good properties. An example is proposed in SAMPE Journal, Vol. 35, No. 3, pp. 58-63 (1999).
In widely known RTM processes, thermoplastic resin is injected under pressure into a reinforcing fiber substrate placed in a closed mold, or a reinforcing fiber substrate placed in an open mold is covered with a vacuum bag, followed by suction for resin injection, the latter process being called vacuum assisted resin transfer molding (VaRTM). Examples of VaRTM are proposed in documents such as U.S. Pat. No. 4,902,215A, U.S. Pat. No. 4,942,013A, and WO01/41993A2. It is reported that VaRTM is suitable for low cost production of large size fiber reinforced composite material.
A variety of thermosetting resins have been applied to RTM, but in particular, epoxy resin and bismaleimide resin are widely used in the field of aircraft manufacturing where high performance materials are essential, with particular importance attached to epoxy resin because of its high cost performance.
An epoxy resin composition to be used in RTM consists mainly of epoxy resin and a hardener, with other additives being added as required. Epoxy resin materials used as main component of an epoxy resin composition for RTM include general-purpose glycidyl ether of bisphenol A, general-purpose glycidyl ether of bisphenol F, novolac glycidyl ether as shown in U.S. Pat. No. 5,942,182A, glycidylamine-type epoxy resin as shown in Japanese Patent Laid-Open Publication (Kokai) HEI 03-050244, diglycidyl anilines as shown in Japanese Patent Laid-Open Publication (Kokai) HEI 03-050242, epoxy resin with a fluorine backbone as shown in U.S. Pat. No. 5,369,192A, epoxy resin with a naphthalene backbone as shown in Japanese Patent Laid-Open Publication (Kokai) HEI 09-137044, epoxy resin with a dicyclopentadiene backbone as shown in WO02/02666, and alicyclic epoxy resin as shown in WO01/92368A1.
Known hardeners generally used with epoxy resin in RTM processes include aliphatic polyamines, aromatic polyamines, acid anhydrides, and Lewis acid complexes. Hardeners widely used with an epoxy resin composition for producing fiber reinforced composite material in the field of aircraft manufacturing include, among others, aromatic polyamine, which is also used frequently as resin for RTM in this field.
Aromatic polyamine materials known to be widely used as resin for RTM include diethyl toluenediamines as shown in U.S. Pat. No. 5,688,877A and WO02/02666A1, aminobenzoic acid esters as shown in Japanese Patent Laid-Open Publication (Kokai) HEI 05-320480, 4,4′-diaminodiphenyl sulfones as shown in Japanese Patent Laid-Open Publication (Kokai) HEI 09-137044, alkyl derivatives of diaminodiphenyl methane as shown in WO02/02666A1, and aromatic diamines with a fluorene backbone as shown in U.S. Pat. No. 5,369,192A. In addition, liquid epoxy resin compositions for filament winding, that consist of glycidylamine type epoxy resin and diglycidyl aniline, plus either diaminodiphenyl sulfone or diaminodiphenyl methane, are disclosed in Japanese Patent Laid-Open Publication (Kokai) SHO 63-077926. Some of said diethyl toluenediamines, aminobenzoic acid esters, and alkyl derivatives of diaminodiphenyl methanes are liquid, while said diaminodiphenyl sulfones, diaminodiphenyl methanes, and aromatic diamines with a fluorene backbone are solid at room temperature.
There are two types, i.e. one part type and two parts type, of epoxy resin products for RTM that consist of an aromatic polyamine. A one part type resin product is a composition that comprises both epoxy resin and aromatic polyamine. For molding, said product is injected after being heated to an appropriate temperature. Since aromatic polyamines are relatively low in reactivity, compositions that consist of epoxy resin and aromatic polyamine can be stored for a relatively long period of time.
A two parts type resin product consists of an epoxy resin based liquid and an aromatic amine based liquid, which are stored separately and mixed to provide a resin composition for molding.
Fiber reinforced composite materials to be used in the field of aircraft manufacturing are generally required to be high in heat resistance. Cured epoxy resin is amorphous and has a glass transition temperature. Above the glass transition temperature, the rigidity of cured resin decreases greatly, resulting in deterioration of mechanical properties of the fiber reinforced composite material. Accordingly, the glass transition temperature of cured resin serves as an indicator of the heat resistance of the resulting fiber reinforced composite material. The glass transition temperature of cured thermosetting resin correlates with the highest temperature found in the heat history of the curing process. In the aerospace industry, curing conditions are frequently set up so that the maximum temperature during the process is about 180° C. ° C.
To use such a high curing temperature of about 180° C., however, molds and other tools have to be resistant to such heat, which increases the required costs. An effective way to reduce the costs for molds and tools is to perform precure at a relatively low temperature in the range of about 80° C. to 140° C., and after demolding, carry out aftercure of the resulting fiber reinforced composite material at about 180° C. In the VaRTM process which requires a vacuum bag, in particular, a low cure temperature makes it possible to use low-price film for vacuum bag, suggesting that low-temperature precure brings highly desirable results.
The reactivity of epoxy resin and aromatic polyamine is relatively low, and cuing at a low temperature will require a longer cure time. So, a catalyst is added to improve the cure cycle. Suitable catalysts for this purpose include BF3.amine complexes as shown in WO01/92368A1, sulfonium salts as shown in U.S. Pat. No. 4,554,342A and Japanese Patent Laid-Open Publication (Kokai) 2002-003581, alkyl esters of strong acids as shown in U.S. Pat. No. 5,688,877A, and polyphenolic compounds as shown in U.S. Pat. No. 4,593,056A.
A two parts type epoxy resin composition is preferred when a catalyst is used. It is because, while the shelf life a one part type epoxy resin composition shortens if a catalyst is added, such a problem can be avoided if a two parts type epoxy resin composition is used. Cured products of epoxy resin compositions designed for production of fiber reinforced composite material to be used in the aerospace industry are required to have many good properties. In addition to the above-mentioned high glass transition temperatures, they should preferably be high in elastic modulus, high in toughness, poor in the glass temperature decrease caused by water absorption (or high in resistance to moist heat), and small in the coefficient of linear expansion. Such aromatic polyamines as 4,4′-diaminodiphenyl sulfone and 3,3′-diaminodiphenyl sulfone can serve to produce cured products with such good properties as small coefficient of linear expansion and high heat resistance, and therefore they are widely used as hardeners to cure epoxy resin compositions for prepreg production in the aerospace industry. However, since diaminodiphenyl sulfones are solid with a high melting point, they are not used in two parts type epoxy resin composition products. It is not impossible in theory to design a batch comprising a solid hardener, but this is not practical because a continuous mixer cannot be applied. Thus, difficulty of using high-performance components has been a major problem with conventional two parts type epoxy resin composition products. Another serious problem with conventional epoxy resin composition products for RTM is trade-off between low viscosity and good properties of cured products. Injection under a relatively high pressure can be performed in the RTM process which uses a closed mold, but the VaRTM process needs a low viscosity at the time of injection because the process uses atmospheric pressure for injection, requiring a considerably low viscosity to carry out impregnation. If precure at 80° C.-140° C. is assumed, furthermore, the inlet temperature has to be set to 40° C.-90° C. An epoxy resin composition to be used should preferably have a viscosity of 500 MPa·s or less at an inlet temperature in this temperature range. Conventional epoxy resin composition products for RTM that can provide cured products with good properties, however, are generally high in viscosity and have to be injected at a high temperature, and therefore, they are not suitable for VaRTM and other low cost processes for which low temperature injection is preferred. A conventional technique that incorporates high temperature injection is shown in International SAMPE Technical Conference, Vol. 31, pp. 296-306 (2000). Said technique uses a mold temperature of 180° C. at the time of injection.
Under such situations, there have been expectations for development of an epoxy resin composition for RTM that has an initial viscosity of 500 MPa·s or less at an inlet temperature in the range of 40° C.-90° C., can be precured at 80° C.-140° C., and can form a cured products that are high in glass transition temperature, elastic modulus and toughness while being small in the glass temperature decrease caused by water absorption and also small in the coefficient of linear expansion.